Carbon dioxide removal from natural gas by membranes in the presence of heavy hydrocarbons and by aqueous diglycolamine®/morpholine

Abstract

Intrinsically defect-free asymmetric hollow fiber polyimide membrane modules were studied in the presence and absence of saturated and aromatic components. Results suggest that an essentially defect-free, non-nodular morphology offers advantages in stability under demanding operating conditions. Earlier work showed serious losses in performance of membranes comprised of similar materials, when the selective layer had a pronounced fused nodular nature as opposed to the intrinsically defect-free skin layers reported on here. Under some conditions for the ternary system, the permselectivity of the membrane is scarcely affected, while under other conditions, permselectivity is negatively affected by as much as 25%. In most cases, for the ternary feeds, significant depression in fluxes was observed due to competition between the CO2, CH4 and heavier hydrocarbons but the effect was even more pronounced for the toluene. In addition to steady state tests in the presence and absence of n-heptane and toluene, modules were conditioned for five days with ternary mixture of CO2, CH4 and one or the other of these heavy hydrocarbons. Following this conditioning process, the modules were studied with a simple binary 10% CO2 /90 % CH4 mixture. These conditioning studies provide insight into the fundamental effects induced in the membrane due to the long term exposure to the complex mixtures. Following exposure to the ternaries containing n-heptane, negligible CO2 permeance increase was seen, while significantly increased permeances were seen under some conditions following toluene exposure even at low pressures of the ternary toluene/CO2/CH4 vii conditioning gas mixture. Although a more protracted process occurs in the case of heptane/CO2/CH4 at 35 0C and 500 ppm, a serious loss in selectivity occurs in the actual ternary tests after exposure for five days. The problem caused by 300 ppm toluene at 35 0C is more immediately apparent, but the ultimate selectivity loss is similar. In addition to the selectivity, in the presence of toluene the permeability is also depressed significantly, presumably due to a greater capability to toluene to compete for added free volume elements introduced in the conditioning process. The permeation enhancement due to toluene exposure is lost slowly when the module downstream is put under vacuum and the gas no longer in contact with the module for up to three weeks. The conditioning treatment has negligible effect at 55 0C, suggesting that that the sorption affinity of toluene decreases with increasing temperature. It is seen from the sorption experiments that penetrant induced conditioning of toluene allows a significant increase in diffusivity than in solubility coefficients, thus allowing for higher permeability and lower selectivity. Solubility, rate of absorption and NMR data were obtained for carbon dioxide in aqueous morpholine (MOR), diglycolamine® (DGA) and aqueous mixtures of MOR and DGA®. Solubility and rate data were acquired in a wetted wall contactor. 23.5 wt%, 65 wt% DGA and 11 wt% MOR/53 wt% DGA concentrations were studied at 298K to 333K. MOR forms an unstable carbamate upon reaction with CO2 compared to DGA which forms a very stable carbamate. Morpholine at 11 wt% of the total amine increases the CO2 equilibrium partial pressure by a factor of 5 to 7 at high loading. The working capacity of 11 wt% MOR/53 wt% DGA was found to be 10% smaller compared to 65 wt% DGA under the conditions studied. The heat of reaction of 11 wt% MOR/53 wt% DGA® was found to be comparable to the 65 wt% DGA. MOR was found also to be more volatile than DGA. The second order rate constant of DGA was found to increase linearly with loading by a factor of 5 over a loading range from 0 to 0.4. Experiments with 65 w% DGA, glycolic acid and potassium formate suggest that viii rate constant increases with loading in the same way as in 65 wt% DGA. The second order rate constant for MOR (k 25C 2=22000 L/mol s) is four times greater than DGA (k 25C 2=6600 L/mol s). The MOR reaction with CO2 was found to follow the zwitterion mechanism; DGA shows zwitterion mechanism in 25 wt% DGA and second order kinetics in 65 wt% DGA. Predictions made with a rigorous eddy diffusivity theory suggests that 11 wt% MOR/53 wt% DGA outperforms 65 wt% DGA of the same concentration by 50 % in terms of CO2 absorption rate. The CO2 enhancement decreases as CO2 loading increases.